Storage: migration + revlog + filtered decks

**Files:** `rslib/src/storage/sqlite.rs` (57%), `storage/revlog/mod.rs` (72%),
`storage/card/filtered.rs` (37%)

Scope:

- **sqlite** — open a collection from an older schema and migrate to the current version
without loss; schema error branches; downgrade path if applicable;
- **revlog** — review-log queries (insert entry, fetch by card, study counts);
- **filtered** — filtered-deck SQL (build the deck, return cards to home deck, ordering).